1. Field of the Invention
This invention relates to magnetic powder having good dispersibility to be utilized for magnetic recording medium, an aqueous coating material containing the magnetic powder which enables coating under a simple and good environment, and also a magnetic recording medium exhibiting excellent magnetic characteristics.
2. Background Information
There has been an increasing demand in the prior art to use magnetic recording medium for uses such as magnetic tape, such as sound recording tape, video tape, etc., floppy discs or magnetic cards; and also various studies have been done to enhance area recording density with an increase in the amount of information to be magnetically recorded.
Whereas, for effecting an increase in magnetic recording density in the coating type recording medium most generally employed at the present time, it is effective to make the magnetic material particles smaller to be fixed by coating onto the surface of a support (e.g., tape) by the use of a binder, and it has been well known in the art that a coating material containing such highly dispersed magnetic powder which enables enhancement of magnetic powder density per unit area of film tape and high orientation will lead to a higher densification of recording.
Some problems have been encountered in employing magnetic fine powder in magnetic recording media.
That is, the magnetic powder to be fixed by coating on the support surface by use of a binder exhibit generally low wettability to a binder solution and also exhibits a magnetic agglomerating tendency and one may consider to impart high dispersibility to the magnetic powder in the binder solution in producing a magnetic recording medium exhibiting a high magnetic recording density as mentioned above. However, in magnetic powder which is frequently used as particles generally of some 10 .mu.m or less, the problem of agglomeration of particles, etc. will occur, and therefore it is not easy to obtain a uniform highly dispersed state in a solution.
In order to solve the problems as described above, for the purpose of improving wettability and magnetic agglomerating tendency for the binder solution and ensuring necessary characteristics as the magnetic recording medium (angular ratio, luster, sensitivity dB, noise dB), it has been generally practiced to add a dispersing agent in the above coating material (binder solution containing magnetic powder). However, the use of a large amount of a dispersing agent for the purpose of poor dispersion accompanied with fine particulation of magnetic powder as described above may cause a deterioration in magnetic characteristics, and also dispersibility of the magnetic fine powder cannot be necessarily improved if even such a large amount of the dispersing agent may be mixed therewith.
Accordingly, for the purpose of improving dispersibility of the magnetic powder, it has been considered to impart high dispersibility in a binder solution by modifying the surface of the magnetic powder.
As the method of such surface treatment for powder is known in general, there may be included utilization of inorganic materials (silicon, titanium, zirconium, aluminum, chromium, etc.), utilization of organic materials (fatty acid, amine, amide, silane coupling agent, etc.), polymerization of monomer on the magnetic powder (acryl, epoxy, etc.) and others. However, in the case of magnetic fine powder which has a strong magnetic agglomerating tendency, surface treatment of good efficiency can be achieved only with difficulty, and therefore there is the problem that it cannot be utilized as such.
Further, the steps of producing the magnetic tape which form the magnetic layer on a base film are broadly classified into the step of forming a coating material in which a coating material is made by dispersing the mixture selected and prepared from various materials as mentioned above into an organic solvent (generally toluene) and the coating step in which the coating material made in the coating material forming step is applied onto the surface of a predetermined substrate. In the latter step, included are the step of passing through the magnetic field for orientation of magnetic powder and the drying step for drying the coating material. Also, in the former coating material forming step, since dispersibility of magnetic powder in water, toluene, etc. has a great influence in the determination of the characteristics of the magnetic tape prepared, its improvement has been sought.
In production of magnetic tapes, etc. of the prior art as described above, an organic solvent such as toluene has been used in the coating material containing magnetic powder as described above, and therefore it is necessary to consider the working environment or waste-liquor disposal instruments in the coating material forming step or the coating step, whereby there is the drawback that these points are reflected in the cost of the product, and there was also the problem of the deleterious influence on the environment.